1. Field of the Invention
The present invention relates to printed circuit board assemblies, and, more particularly, to mounting components to printed circuit boards.
2. Description of the Related Art
Printed circuit boards (PCBs) are a widely used component in all types of electronic devices. For example, PCBs are commonly used as motherboards or system boards for computer systems, especially personal computer systems. PCBs eliminate the need for a chassis containing tangled wire connections and are relatively easy and inexpensive to manufacture, install, and use. As a result, complex electronic equipment such as personal computer systems can be made less expensively and much more compactly.
PCBs generally include a board constructed from an insulating material such as a glass fiber reinforced epoxy resin. The board can be rigid or it can be relatively flexible. A plurality of electrically conductive circuitry tracks are formed onto the board by printing techniques. These tracks are then connected to various electrical components when the components are affixed to the PCB to provide a printed circuit board assembly (PCBA). The term PCB is often used to include the entire PCBA.
Components are increasingly being affixed to PCBs by the use of surface mount technology (SMT) as opposed to through-hole packaged-component technology. In through-hole mounting, a component lead is inserted into a hole on a PCB and soldered into place. SMT, on the other hand, terminates packaged circuit components in a planer manner on the surface of the PCB.
The use of SMT permits relatively smaller component packages to be mounted to PCBs. Also, SMT does not usually require component mounting holes in the interconnecting substrates (i.e., PCBs). Consequently, smaller assemblies and ultimately smaller equipment may be achieved through increased component placement densities, finer-pitch component terminals, and the ability to have components mounted on both sides of the assembly. Further, SMT provides improved shock and vibration environmental stability through the use of smaller (lower mass) component packages.
Conductive pads are typically used to connect components to the PCB. Each pad is usually rectangular in shape and sized in relation to the particular lead which is to be soldered to it. Three-lead surface mount discretes typically include a larger lead that covers a large area underneath the body of the component. (Exemplary three-lead surface mount discretes include the DPAK, TO-252 or CASE 369A-10 industry standard package and the larger D2PAK (or D.sup.2 PAK), TO-263 or CASE 418B-01 industry standard packages.) The larger lead on the three-lead surface mount discretes serves both functionality and structural purposes. Pads for a three-lead SMT discrete can also be large relative to the size of the component. The size of the surface area of the component lead coming into contact with the PCB dictates the size of the pad used. Relatively large pads can cause the component to slide or skew on the pad due to solder buildup. This can cause component collisions on high density PCBs as well as a less than optimal solder joint.
Referring to FIG. 1, a portion of PCB 100 is shown. PCB 100 includes SMT pads 110, 120 and 130. Pad 110 is a rectangular pad having a horizontal width of approximately 420 mils (i.e., 0.42 inches) and a vertical length of approximately 348 mils, as oriented in FIG. 1. Pads 120 and 130 are approximately 46 mils by 135 mils. Pads 120 and 130 are approximately 180 mils from the edge of pad 110. Pads 110, 120, 130 are used to solder the leads of a component to PCB 100.
Referring to FIG. 2, a portion of PCB 100 is shown with component 200 soldered onto PCB 100. In this embodiment, component 200 is a D2PAK component. Component 200 includes side leads 210 and 220 which are soldered to pads 120 and 130, respectively. Component 200 further includes base lead 230. Base lead 230 protrudes approximately 50 mils from the upper edge of the body of component 200. Base lead 230 is soldered to pad 310.
As shown in FIG. 2, component 200 is a three-lead, surface-mount discrete which is exemplary of surface mounted components. Other types of surface mounted components are mounted to PCB 100 using analogous pads and leads.
To mount component 200 to PCB 100, solder paste is placed on pads 110, 120 and 130. The component is placed on the solder paste. Heat is applied to melt the solder paste. Component 200 floats on a thin bed of melted solder. Solder builds up at the edges of the leads, e.g., where the edge of base lead 230 meets pad 110 and at the edges of leads 210 and 200. When the solder cools, it solidifies and binds lead 230 to pad 110 and leads 210 and 220 to pads 120 and 130, respectively. Where the solder was built-up, a solder joint, or fillet, is formed. Component 200 is mounted to the surface of PCB 100 by solder fillets binding leads 210, 220, 230 to pads 120, 130, 100, respectively.
When the solder is solidifying, it contracts and pulls the component. There is a strong pull in the upper direction (as the component is oriented in FIG. 2) because there is a long (component 200 body length) solder fillet pulling in that direction. A solder fillet exists all along the edge of lead 230, pulling component 200, and therefore leads 210 and 220, away from pads 120 and 130. Component 200 can be skewed toward the edge of lead 230.
When component 200 is skewed to a side, the contact between leads 210, 220, 230 and pads 120, 130, 110 may not be as effective as desired. Also, if component 200 can be skewed to a side, it could collide with other components placed a certain distance away from component 200.